Custom Save Data only has to do with how you modified the character in the Custom Tab - stuff like: skin color, eye color, breast size, etc... and generates that data for you to copy-paste-save to a .txt file for future reference...but I wouldn't bother with that if I were you.
You should just use any preferences of what you did in the Custom Tab, and use the "Store Custom Char" in the Modding Tab, use the "Save Character" option, and just type some name for your character and hit the "Save Character" button; and if you have a Background image and/or Hair image you want to save along with your character, make sure those images are in the same folder/location as the actual SuperDeepthroat game (the custom character will be saved in the "Scene" Tab, under the "Her - Custom" image button)
Hair imports are usually: .png files (Konashion blog Homepage - Links - Booru (for starters)
backgrounds are usually: jpg files (seach the SuperDeepthraot Mods forum for "background" using this websites's search option)
swf imports are usually: well, they're usually swf files (good luck).